Galactic Thief Almost Got Away with It

One of the closest galaxies to the Milky Way almost got away with theft. However, new simulations convicted the Large Magellanic Cloud (LMC) of stealing stars from its neighbor, the Small Magellanic Cloud (SMC). And the crucial evidence came from surveys looking for something entirely different -- dark objects on the outskirts of the Milky Way.

Astronomers have been monitoring the LMC to hunt for evidence of massive compact halo objects, or MACHOs. MACHOs were thought to be faint objects, roughly the mass of a star, but their exact nature was unknown. Several surveys looked for MACHOs in order to find out if they could be a major component of dark matter -- the unseen stuff that holds galaxies together.

In order for MACHOs to make up dark matter, they must be so faint that they can't be directly detected. Instead, astronomers looked for a phenomenon known as microlensing. During a microlensing event, a nearby object passes in front of a more distant star. The gravity of the closer object bends light from the star like a lens, magnifying it and causing it to brighten.

By studying the LMC, astronomers hoped to see MACHOs within the Milky Way lensing distant LMC stars. The number of microlensing events observed by various teams was smaller than needed to account for dark matter, but much higher than expected from the known population of stars in the Milky Way. This left the origin of the observed events a puzzle and the existence of MACHOs as exotic objects a possibility.

"We originally set out to understand the evolution of the interacting LMC and SMC galaxies," explains lead author Gurtina Besla of Columbia University. "We were surprised that, in addition, we could rule out the idea that dark matter is contained in MACHOs."

"Instead of MACHOs, a trail of stars removed from the SMC is responsible for the microlensing events," says co-author Avi Loeb of the Harvard-Smithsonian Center for Astrophysics.

"You could say we discovered a crime of galactic proportions," he adds.

Computer simulations showed that the most likely explanation for the observed microlensing events was an unseen population of stars removed by the LMC from its companion, the SMC. Foreground stars in the LMC are gravitationally lensing the trail of removed stars located behind the LMC from our point of view.

Only a fast-moving population of stars could yield the observed rate and durations of the microlensing events. The best way to get such a stellar population is a galactic collision, which appears to have occurred in the LMC-SMC system.

"By reconstructing the scene, we found that the LMC and SMC collided violently hundreds of millions of years ago. That's when the LMC stripped out the lensed stars," says Loeb.

Their research also supports recent findings suggesting that both Magellanic Clouds are on their first pass by the Milky Way.

Although the evidence for the trail of lensed stars is persuasive, they haven't been directly observed yet. A number of teams are searching for the signatures of these stars within a bridge of gas that connects the Magellanic Clouds.

The simulation results will be published in the Monthly Notices of the Royal Astronomical Society and are available online at http://arxiv.org/abs/1205.4724

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

One of the closest galaxies to the Milky Way almost got away with theft. However, new simulations convicted the Large Magellanic Cloud (LMC) of stealing stars from its neighbor, the Small Magellanic Cloud (SMC). And the crucial evidence came from surveys looking for something entirely different -- dark objects on the outskirts of the Milky Way.